1. Field of the Invention
The present invention relates to packaging for semiconductor integrated circuits and, in particular, to a plastic package design that uses a non-quadrangular die attach paddle to provide heat dissipation capability comparable to that of a plastic package with a heat spreader, but at lower cost.
2. Discussion of the Related Art
Currently-available high-speed, high-density integrated circuit devices can generate large amounts of heat. The heat generated by these devices must be removed because the normal integrated circuit failure mechanisms are usually enhanced by increased temperature. Therefore, to prevent reliability and performance problems, the temperature of integrated circuit chips must be kept within specified limits.
FIG. 1 shows a design of a conventional copper lead frame structure 10. The lead frame structure 10 includes a supporting skeleton (not shown) that serves to support the integrity of the remaining elements of the structure 10 during the attachment of a semiconductor die and other manufacturing steps. Typically, the lead frame skeleton supports die attach and lead structures for a plurality of integrated circuit devices. The lead frame skeleton may be in the form of a lead frame strip or a reel-to-reel lead frame configuration for automated processing.
As further shown in FIG. 1, the conventional lead frame structure includes a die attach paddle (DAP) 12 that is usually formed as a solid square or, in some circumstances, can be a solid rectangular. The DAP 12 is attached to the lead frame skeleton by four tie bars 14 that extend from the corners of the quadrangular DAP structure 12. The lead frame structure also includes a plurality of leads 16 with each lead 16 having an outer edge 18 connected to the lead frame skeleton and an inner edge 20 disposed adjacent to but separated from the periphery of the die attach paddle 12. In the FIG. 1 design, the space between the edge of the DAP 12 and the closest inner lead edge 30 is 0.044 inches. FIG. 1 shows a so-called high lead count package that includes 160 leads 18, 40 leads 18 disposed on each side of the solid square die attach paddle 12.
Typically, the lead frame structure 10 will also includes dam bars (now shown) which maintain the physical integrity of the leads 16 during processing, but which are removed before the completion of the final packaging process in order to provide individual electrical integrity to the leads 16.
Die attach techniques other than the solid DAP design described above have been utilized.
For example, Motorola's U.S. Pat. No. 4,924,291, issued May 8, 1990, discloses a so-called "flagless" semiconductor package in which the die attach paddle is completely eliminated and the semiconductor die is mounted directly to the inner ends of the lead frame tie bars. The Motorola '291 patent also discloses an embodiment in which a guard ring is utilized to maintain the structural integrity of the bars to which the semiconductor die is attached.
FIGS. 2-5 show other known die attach systems. In each of FIGS. 2-5, the semiconductor die is shown in "dashed" lines.
The system of the Motorola '291 patent and those shown in FIGS. 2-5 differ fundamentally from the FIG. 1 die attach system in that these systems do not utilize a "solid" quadrangular die attach paddle, i.e. a substantially flat four-sided DAP that is substantially devoid of openings within its four sides.
In plastic packages, the integrated circuit chip is mounted on the die attach paddle, wire bonded to the inner edges of the individual leads 16 and encapsulated in plastic molding. To meet temperature specifications, a heat spreader (not shown) can be placed under the chip to improve heat dissipation. Usually, the heat spreader is also copper, which increases the cost of the packaged product. Use of heat spreaders can also cause reliability problems.
It would, therefore, be desirable to enhance heat dissipation in integrated circuit devices, but at low cost, particularly in high pin count devices.